In telecommunication systems for civilian use, with special reference to mobile telephones, there is a problem of providing microwave filters that, placed along a transmission line, allow the is separation of different band or frequency channels; for example, separating transmission channels from receiving channels.
Usually these filters are implemented with a plurality of cavities in cascade and are mutually coupled through irises, screws or the like. As is known, these cavities, which may be of the waveguide type with a cylindrical or prismatic shape, or of the co-axial type, with an internal metal conductor, are of a size that depends on the wavelength of the signal to be filtered, therefore the filter obtained may be quite large, especially at lower frequencies (1-4 GHz), and as a consequence the resulting overall dimensions may be excessive.
This problem becomes more critical when the telecommunications system development is such as to make a considerable number of these filters necessary, especially when these are fitted near aerials, often installed on the roofs of civil buildings.
One method of reducing the size of these filters, which has become common in recent years, is to insert a block of dielectric material into each cavity.
Because of the high permittivity of the material introduced into the resonator, the electromagnetic field remains mainly concentrated inside, and thus the dimensions of the cavity, calculated to obtain the resonance at a certain wavelength, are considerably reduced. In fact, the dimensions of an equivalent filter with dielectric-loaded resonators are reduced from between one third to one sixth of the original volume. The electrical characteristics of the filter are not excessively penalized, because of the availability of low loss, high temperature-stability ceramic materials.
Another method of obtaining small sized filters is to reduce the number of cavities used, exploiting two or more resonant modes in each cavity by means of the re-use technique, which permits the design of dual mode or triple mode resonators. The coupling between the modes is obtained by perturbing the cavity section in the diagonal plane in relation to the polarization planes of the modes themselves. The effect that results is the same as that which can be obtained with two ordinary cavities, thus a filter with a desired band can be obtained with half the number of cavities.
Moreover, the re-use of the same cavity also permits more sophisticated transfer functions than transfer functions with all the infinite or polynomial transmission zeroes, characteristic of a cavity plurality simply connected in cascade.
One of the problems found in the preparation of filters that use cavities of the type mentioned, is the difficulty in obtaining couplings with a sufficiently high value, especially when the band pass required is comparatively wide, e.g. more than one percent of the central frequency.
It is a known fact that cavity couplings are obtained by the introduction of mechanical elements, such as probes or screws, the latter also permitting the tuning of the same. Obviously, if the cavity contains dielectric material inside, there are further difficulties in the arrangement of these elements. In fact, the dielectric material, on one hand makes stronger the internal electromagnetic field, limiting the peripheral field that intervenes in the couplings, on the other hand it mechanically limits the penetration of the screws and probes.
The problem becomes worse due to the fact that all these elements are to be preferably located on the plane which is perpendicular to the rotation axis of the dielectric material and divides it into two equal parts: in fact, in this way the operation is carried out where there is a high electromagnetic field, obtaining a coupling of a greater value, and the energizing of spurious resonating modes is avoided, which could generate anomalous responses in the operating band.
Furthermore, when the filter is designed to function at very low frequencies, for example between 1 and 4 GHz, where the wavelength, and therefore also the size of the cavity, is greater, the cavity internal volume has to be occupied as much as possible by the dielectric material, so as to obtain the maximum reduction in the overall dimensions. As a consequence, the space to house screws and probes is further limited.
Among the dielectric loaded cavities known at present, is that described in U.S. Pat. No. 5,008,640, issued in the United States on Apr. 6, 1991, entitled “Dielectric-loaded cavity resonator”, in the name of the present applicant, which solves the problem arising from the dimensions and has low losses in the pass band. However, it is not suitable for broadband filters, which require very tight couplings between resonators and therefore considerable penetration of the coupling elements in the dielectric resonator transverse symmetry plane.
Another known cavity is that described in WO 99/19933 published on Apr. 22, 1999, in the name of Filtronic PLC, entitled “Composite resonator”. In the resonator described, the dielectric element rests on the base of the metal cavity and has a metal disk on the summit. This configuration permits a considerable reduction in the presence of spurious modes in the vicinity of the filter operating frequency, but increases the resonator losses. Further-more, to obtain the required couplings, certain mechanical devices are necessary, such as plates and disks with a rather critical adjustment.